Ieuan Williams, a senior maritime operations officer at HM Coastguard, said: “The explosive ordnance disposal team plans to detonate the ordnance at 6pm today. Until that time we have taken measures to … clear the area of vessels to keep the public safe.”

The bomb was reported in the early hours of Wednesday by a diving team from the Hinkley Point plant. They were clearing the seabed for intake and outtake pipes for cooling water for the reactors on the Hinkley Point C plant. It is the third suspected second world war bomb to be found in the Bristol Channel in the past three weeks.

On 8 August, a 500lb device was discovered 2.5 miles from the coast. On 16 August, a 250lb bomb was found less than half a mile from the power station. Both were destroyed in controlled explosions.

Hinkley Point, near Bridgwater in Somerset, houses two power nuclear power stations run by the French company EDF: Hinkley Point B, scheduled to be decommissioned in 2023; and Hinkley Point A, which closed in 2000.

Hinkley Point C will be the UK’s first new nuclear plant for more than two decades.

David Eccles, EDF’s head of stakeholder engagement for the project, said: “It is normal practice to check the seabed before construction activity starts on any marine project.

“The safety of the public and our workforce is our priority, and we have a team of 10 divers checking the seabed ahead of the construction of the main cooling water tunnels and associated seabed structures for Hinkley Point C.”

An EDF source conceded that divers could find more unexploded ordnance before the exercise to clear the area was completed, as the channel was used as a former army training range.

The project to clear the seabed is expected to take several more weeks.